Mental Ray's ambient occlusion pass isn't rendering correctly

I'm trying to render an ambient occlusion pass for a shot that involves a security camera model built into a wall like so (http://puu.sh/jgpTa/a7b5f42481.jpg) <-- that's just the beauty pass and the shadow pass so it's missing the ambient occlusion pass.

The security camera model was downloaded from TurboSquid and the scene file was in V-ray. So the first thing I did to get started with the ambient occlusion pass was to apply a standard material to everything in the scene, which includes the security camera and the wall, to make sure that all the vray materials are gone from the scene. Then I did a material override and dragged the ambient occlusion material just like how I usually do. As far as I know, when I render an ambient occlusion pass from mental ray, the whole image becomes white except the contact shadows. However, this is what I got (http://puu.sh/jgqoJ/324f6e9e9b.jpg). I made sure every object was visible to the camera but the wall behind the camera appeared to be ignoring the ambient occlusion material override.w